Iron oxide materials exist naturally as several minerals. These minerals include red, yellow, brown, and black iron oxide materials. For example, red iron oxide minerals are usually hematite (.alpha.-Fe.sub.2 O.sub.3) which has a hexagonal crystal system and occurs in several well known habits. Yellow iron oxide can be lepidocrocite (.gamma.-FeOOH or Fe.sub.2 O.sub.3.nH.sub.2 O) or goethite (.alpha.-FeOOH or Fe.sub.2 O.sub.3.nH.sub.2 O) which have an orthorhombic crystal class and may occur in a variety of habits. Brown iron oxide is maghemite (.gamma.-Fe.sub.2 O.sub.3) which is dimorphous with hematite. Black iron oxide is magnetite (Fe.sub.3 O.sub.4) which has a cubic crystal system and may also be found in a number of habits. Brown and black iron oxide arc often magnetic.
Iron oxide is an important component in pigments, catalysts, magnetic recording and storage media, and many other applications. While much work has been done in the development of these applications, determining which parameters of a given iron oxide material are responsible for a given characteristic making it desirable for that particular application is not readily amenable to theoretical treatment. It is known that the shape, size, and crystal structure of the particles formed by iron oxide are important or even determinative of their properties. However, the precise nature of the relationship among these characteristics and their resulting properties is most often established empirically.
Synthetic hematite, goethite, lepidocrocite, and magnetite arc among the most important iron oxides for use in industrial applications. Synthetic hematite produced by calcination of synthetic goethite is most widely used to catalyze the conversion of ethylbenzene to styrene because these materials often have the highest purity (&gt;98%Fe.sub.2 O.sub.3).
Synthetic hematite may take on several different particle habits depending upon the process in which it was made. Acicular (needle shaped) synthetic hematite particles may be obtained by calcination of yellow iron oxide produced by the Laux process for aniline manufacture. Branched acicular particles may be obtained by calcination of synthetically produced goethite. Random spheroidal synthetic hematite may be obtained from the Ruthner process for regeneration of spent steel mill "pickling" acid. Synthetic cubic hematite particles may be obtained by calcination of synthetic magnetite.
Certain catalytic substances can undergo significant changes in surface structure under differing conditions. This can have a profound impact on the electronic and chemical properties of the substances including their catalytic activity. For example, some such changes can be adsorbate induced and such changes are largely directed to forming a more thermodynamically stable adsorbate-surface configuration. Typically, such restructuring of the surface occurs in cluster-like fashion. That is, the effect is largely localized on the surface to which the adsorbate adheres. It is also known that certain substances can be used to modify the structure of a catalyst or promote its selectivity or activity. An example of this is found in the addition of alumina to iron catalysts for use in the catalytic synthesis of ammonia. The addition of alumina probably results in a restructuring of the iron compound which is likely a chemical effect due to the formation of an iron aluminate.
U.S. Pat. Nos. 4,052,338; 4,098,723; 4,143,083; 4,144,197; and 4,152,300 all propose dehydrogenation catalysts comprising small amounts of oxidic compounds and rare earths added to iron-potassium oxide base catalysts. In each case, these components were blended, pelletized, and dried. The pellets were then calcined. Selectivity was consistent at approximately 92 mole % (for styrene) among these compositions at a 70% molar conversion of ethylbenzene to products.
It has now been found that iron oxide compositions can be restructured to prepare particles with low surface area and uniquely modified habits. They are particularly useful as catalysts. Catalysts comprised of these compositions have enhanced selectivity in the reactions they are used to catalyze.